1. Field of the Invention
The present invention relates to a cover sheet for placing on a rotatable grinding/polishing (treatment) disc for manual, automatic or semiautomatic preparations of materialographic samples. The invention also relates to a method of performing preparation of a materialographic sample using one or more cover sheets.
2. The Prior Art
The preparation of materialographic samples, e.g., metallographic, petrographic and ceramographic samples, is usually performed by use of a rotatable grinding/polishing disc covered with a grinding/polishing sheet. The preparation is normally performed in 3 or more steps, viz. one or more plane grinding steps, one or more fine grinding steps, and one or more polishing steps.
Traditional grinding sheets also designated cover sheets, contain an abrasive material incorporated into the surface adapted to be brought into contact with the sample to be treated. A very used type of cover sheet consists of a paper or paper-like substrate onto which abrasive particles are adhered. The main drawback of this sheet type is, that it has a very short lifetime and needs to be exchanged very often, which requires a lot of work.
Some of the cover sheets employed today are not in themselves abrasive, but serve as a substrate for an abrasive grinding/polishing agent, which is sprayed or otherwise applied onto the surface of the cover sheet, either automatically or manually before or under the grinding/polishing process.
Cover sheets in the form of metal support plates covered with hard coatings consisting of a composite material, e.g. of epoxy with hard particles of one or more metals or ceramic particles embedded therein are also well known. In the prior art, cf. e.g. DK 143096, such coatings are moulded on or glued to a self-supporting rigid plate, e.g. in the form of mutually separated segments, where after the surface of the coatings is straightened by turning and/or grinding. In this manner a tool is formed, which can be mounted as grinding/polishing disc in a grinding/polishing apparatus.
WO 96/07508 publication discloses a similar cover sheet consisting of a thin foil, which is relatively stiff and onto which a hard composite material has been applied through a stencil in a desired thickness and pattern.
When performing preparation of materialographic samples using prior art grinding/polishing sheets as described above, the operator must in the first plan grinding step use a first cover sheet containing incorporated abrasive particles in order to remove sufficient material relatively fast to make the surface plan. Thereafter, for the fine grinding of the samples, he may use a non-abrasive cover sheet together with an abrasive grinding agent or he may use a cover sheet containing incorporated abrasive particles with has a smaller size than the abrasive particles in the first cover sheet. In case the cover sheets used for the fine grinding procedure is in the form of SiC paper with incorporated abrasive particles, he will need to employ several fine grinding step with progressively finer grain sizes e.g. 2-4 fine grinding step, and in each of these fine grinding step he need to use several sheets due to weak durability,. In the polishing step, he should replace the cover sheet with a cover sheet adapted for polishing in combination with the use of a polishing agent. If the sample should have a very fine polished surface e.g. an oxide polished surface, he will need to change the sheet once more and replace it with a cover sheet adapted for performing an oxide polishing.
As it appears, the operation is very time-consuming, and it is very expensive to construct an apparatus, which automatically can propagate the samples due to the many shifts of cover sheets. If the preparation is conducted manually, it is very time-consuming for the operator due to the exchange of cover sheets or the operator may use several rotatable discs, one for each sheet. Generally, it is an expensive solution to use several rotatable discs.
U.S. Pat. No. 959,054 describes a cover sheet containing abrasive material to one of its surfaces. The abrasive on the sheet is of different degree of fineness varying gradually and uniformly from coarser abrasive material at the centre, to finer abrasive material at the outer edge.
When using this cover sheet, the article to be treated is placed against the sheet near its centre and gradually moved towards the outer edge. The main drawback of using this sheet is that the abrasive material is worn of as the grinding process is performed. Some of the larger abrasive particles worn out of the sheet from near its centre will be incorporated into the sheet or be grabbed by the smaller abrasive particles closer to its outer edge, and consequently, be an obstruction of a polishing of a sample, because these coarser particles will create unwanted grinding scratches in the sample. Generally, it is almost impossible to control the grinding/polishing procedure because the abrasive materials after the procedure is initiated will be more or less mixed op with each other. Another drawback is that the sheet mainly will be worn in the annular centre portion of each abrasive zone, which means that it can only be used for a very short time before a height difference is created. Such height difference in the abrasive material may result in an uneven grinding and polishing of the samples, which is highly unwanted. As a consequence of these many drawbacks of the cover sheet described in U.S. Pat. No. 959,054, this sheet has never been commercialised.
The objective of the present invention is to provide a grinding/polishing cover sheet, by which the drawbacks of the previously known cover sheets are avoided.
It is particular an object of the invention to provide a grinding/polishing cover sheet by use of which, the numerous changes of cover sheets during a process of preparation of a materialographic sample can be reduced or avoided, and particularly, it is an objective to provide a cover sheet by use of which two or more grinding/polishing steps can be performed without any substantial risk of contaminating abrasive materials from a first grinding/polishing step to a second grinding/polishing step under performing of this second step.
Another objective of the present invention is to provide a grinding/polishing cover sheet, which can be easily used without the risk of creating height differences in the surface of the sheet, which might result in creating the surface of a sample uneven.
Yet a further objective of the present invention is to provide a method of grinding/polishing a sample in two or more steps using one single cover sheet, by use of which method the risk of contaminating abrasive materials from a first grinding/polishing step to a second grinding/polishing step under performing of this second step is substantially eliminated.